How to Optimize Operations With the Right Gears
Identifying the Right Recipe for the Application is Key
By Brian Dengel, General Manager at KHK-USA
In all packaging machinery, whether it is a pouch filler, labeling equipment or sealing equipment, motion begins at the motor and needs to travel to all elements of the machinery. Gears are an economical and efficient solution to transmit this motion. Gears come in many forms and many materials.
Different materials make gears suitable for the desired application. The key involves identifying the right recipe:
For linear applications, gear racks are used. They are most commonly produced from carbon steel. This is a very suitable material for racks in most environments and under most use conditions. The reason that this material works well in packaging applications is due to the relative strength of the rack tooth in mesh with the teeth on the pinion.
In almost every application, the pinion is inherently the weaker member in the mesh and will fail long before the rack. Carbon steel lends itself to allowing the gear racks to be heat-treated for additional strength, it permits the machining of additional features such as threaded or bolt holes, it maintains dimensional stability, and can be straightened if necessary.
Packaging machinery with requirements for high loads, high speeds, or high accuracy require ground gear racks. The best material for ground gear racks in these applications is alloy steel. Alloy steels are typically carburized, and ground finished, in order to add additional surface durability and precision. As with carbon steel racks, threaded holes or bolt holes can be added to allow for attachment.
For machinery that will be subjected to frequent wash-downs or in food processing environments, stainless steel is frequently used for gear racks. Most stainless steels share all the attributes of carbon steel except for the ability to be heat-treated for additional strength. Some stainless steels are also non-magnetic which is necessary in some packaging applications.
In some applications, if there is a need for the rack to be self-lubricating, or if there is a need to reduce system weight, a plastic rack can be used. Plastic racks are exceptionally durable and useful, only when selected for the proper environment. For example, acetal racks will not maintain their straightness over a long length (greater than 1 meter) and are known to have the possibility of voids within the material. The possibility of voids in the rack makes the machining of bolt holes or threaded holes a risky venture.
Alternately, nylon racks will not maintain their dimensional accuracy when exposed to varying temperatures or changes in humidity, as this material reacts to both temperature and moisture. For a direct replacement, based on the relative tooth strength, a nylon gear rack will need to have a face width or pitch that is six times larger than that of a carbon steel rack in order to manage the same loading.
For applications where a reduction in speed is needed, worm gearing is a suitable gearing mechanism. When choosing a worm gear pair, it is critical that the drive gear have a surface strength (durability) that is greater than that of the driven gear. It is because of this requirement that most worms are made from carbon or alloy steel and most worm wheels are made from bronze, cast iron, or aluminum bronze.
Consider a 60:1 worm gear pair. In this arrangement, the worm will be rotating one tooth in and out of mesh continuously. However, the worm wheel will be engaging just one tooth every revolution. Therefore, each tooth of the worm wheel has a duty cycle of one worm revolution engaged and 59 revolutions disengaged. This implies that the worm needs to have a durability 60 times greater than that of the wheel.
If the wheel was produced from a material that has the same durability as the worm, the worm will fail in an accelerated manner due to scoring. This is a condition that occurs when metal-to-metal contact causes the tooth flanks of the gears to weld together. This process pulls metal from the pair and begins to scratch the tooth surface in the sliding direction. Although proper lubricant can minimize scoring, using materials that allow for heat dissipation is the better design consideration.
Change in direction
For packaging applications where a change in direction is needed, bevel gears are typically used. These gears are suitable in high torque environments and are usually produced from carbon steel, stainless steel, or alloy steel. Bevel gears can be produced with either straight teeth or spiral teeth.
Carbon steel and alloy steel are used to produce both styles as these materials can be heat treated and ground finished. Stainless steel is typically used for only for straight bevel gears. Carbon steel bevel gears are typically used in lower speed, lower torque, commercial applications; whereas alloy steel bevel gears are designed for high speed, high torque and precision applications.
However, there are applications where acetal or nylon bevel gears are suitable. Plastics can be used to produce straight tooth bevel gears, but they cannot be used to produce spiral bevel, zerol bevel or hypoid bevel gears, as the material would deform because of the heat generated by the cutting process.
Spur gears are the most popular form of gearing and can be produced in almost any material. Gears produced 100 years ago were typically produced from cast iron. This was an economical method for producing change gears for industrial equipment.
Selecting gear materials
Materials commonly used today are several types of steel, plastic, and specialty metals:
Carbon steel gears are economical and suitable for many packaging applications. Carbon steels offer moderate strength, low cost and ease of machinability.
Stainless steels also offer moderate strength and ease of machinability but at a higher material cost. Some stainless steels are also non-magnetic. Alloy steels offer superior strength and can be hardened for superior durability but at a high raw material cost. All carbon steels, some stainless steels, and most alloy steels can be heat-treated to improve surface durability. Since all steels are dimensionally stable, gears produced from steel can have keyways and tapped holes added to the hub in order to rigidly fix them to the motor shaft.
Acetal gears are typically the most economical spur gear available, as they are generally produced by injection molding. The moderate cost of raw material and low cost of machining make this a common material for prototyping. Acetal is a very dimensionally stable plastic due to its low absorption of moisture. It is recommended that acetal gears are not subjected to shock loading as they have a poor resilience to impact loading.
Nylon gears are more common in packaging applications due to the material’s moderate cost, the ease of machining, the material’s self-lubricating properties, and its vibration absorbing abilities. Nylon gears are sometimes made from reinforced nylon. Depending on the reinforcing material, a nylon gear can improve its bending strength by 30 to 90%. Fiberglass-filled nylons maintain the self-lubricating and weight saving properties, reduce the dimensional growth due to temperature and moisture, and increase the bending strength by 30%, albeit at a 50% increase in raw material cost.
Carbon fiber-filled nylons also maintain the self-lubricating and weight saving properties, greatly reduce the growth due to temperature and moisture, and increase the bending strength by 90%. However, the raw material cost is at least 600% more expensive than unfilled nylons.
Unfilled nylon gears have a bending strength which is one sixth that of a similar sized carbon steel gear. This reduced torque carrying capacity is useful in the design of safety mechanisms within gear drives. When designing a drive system for a packaging machine, it can be useful to include a nylon gear so that the gear can fail purposely if the torque of the system is exceeded and in turn keep the other components of the drive from being damaged. Many complex gear systems that are not supposed to be driven in reverse will include a sacrificial nylon gear for this reason.
Aluminum gears are used in hand-controlled applications. Aluminum has a high strength to weight ratio which allows for its use in the production of fine pitch gearing. This makes it suitable for low torque, low speed, instrument drive type applications. The major disadvantage of using aluminum for gearing is that it has a high coefficient of thermal expansion when compared to steel.
As detailed in the examples above, there are many distinct materials for each style of gearing. Each material has unique features and properties that make it specifically suited for a particular application. The machinery that you are designing will dictate the material that you should be selecting and should be incorporated into the design process, just as the type of gearing needed to accomplish the task at hand.
About the Author
Brian Dengel is general manager of KHK-USA, which is based in Mineola, New York. Learn more online at www.khkgears.us.